Conventionally, an automotive vehicle having a navigation apparatus is equipped with an antenna device for receiving a GPS radio wave and for inputting a received GPS signal to the navigation apparatus (i.e., a GPS receiver in the navigation apparatus) through a wiring such as a co-axial cable.
The antenna device has, in addition to a GPS antenna element for receiving the GPS radio wave, an amplifier for amplifying the GPS signal in the GPS radio wave to a certain level (see Japanese patent document JP-A-2008-5468, paragraph [0003], for example).
When the antenna device is used in the vehicle, a power supply for the amplifier is controlled by the same switch (e.g., an accessory switch in the vehicle) for controlling the power supply for the navigation apparatus for the purpose of synchronous operation of the amplifier and the navigation apparatus.
On the other hand, a data communication module (DCM) is known as a communication device that is used in the vehicle for sending and receiving various information to and from a station, in order for the station to monitor the vehicle, or to send, to the vehicle, information that is required for vehicle control (see Japanese patent document JP-A-2006-128807, paragraph [0015], for example).
Further, the DCM may have a function for position detection that is used in, for example, a cellular phone to send the position information of the vehicle to the station. That is, a technique for locating a current position on the station side (a so-called gpsOne technique (Registered Trademark)) by sending data in the GPS signal to the station, in case that the current position of, for example, the cellular phone cannot be detected based on the GPS signal received through a built-in GPS antenna element (see Japanese patent document JP-A-2008-22492, paragraph [0030], for example).
By applying the above-described technique to the DCM for use in the vehicle, an accurate position of the vehicle is notified to the station in case of an accident, or a theft even when the GPS signal cannot be properly received by the GPS antenna element.
Therefore, when both of the DCM having the position detection function and the navigation apparatus are disposed on the same vehicle, the GPS antenna element for the navigation apparatus and the GPS antenna element for the DCM are also disposed on the same vehicle.
Both of the GPS antenna elements have the same resonance frequency for utilizing the same frequency band of the GPS radio wave. Therefore, if both of the two antenna elements are disposed at positions close to each other, the antenna elements influence with each other due to the overlapping of the resonance frequency band.
When the two antenna elements have to be used in one vehicle, in order to avoid the mutual influence, the GPS antenna element for the communication device is, for example, disposed on the roof of the vehicle, and the GPS antenna element (having the amplifier combined therein) for the navigation apparatus is disposed in an inside of the compartment of the vehicle. That is, by disposing the two antenna elements apart from each other in the same vehicle, the reception characteristics of the desired level are achieved by avoiding the mutual influence caused by interference.
In recent years, due to the antenna integration in progress, the antenna for the DCM communication device and the antenna for GPS radio wave are usually disposed as a single antenna device integrated in one package.
Further, in a sheer pressure of the antenna integration of these days, the DCM antenna device packing, in one package, both of the communication antenna element and the GPS antenna element is requested to be combined with another antenna device for the navigation apparatus.
Therefore, both of the two GPS antenna elements for the DCM and for the navigation apparatus are disposed at close positions in one package, with compromise, by allowing the performance deterioration, because the antenna integration of combining these different types of antennas in one package deteriorates, so far, the performance of the antenna due to the interference between the two GPS antenna elements using the same frequency band. In this case, the C/N (Carrier-Noise) ratio of the GPS signals to be input respectively to the navigation apparatus and the communication device (i.e., the DCM) is deteriorated, thereby compromising the measurement accuracy of each of the DCM and the navigation apparatus.
Alternatively, a single GPS antenna element may be shared by the navigation apparatus and the communication device for solving the above problem. However, the shared use of the single GPS antenna element by the different devices leads to the distribution loss that also deteriorates the input level of the GPS signal for each of the two devices if the GPS signal is simply distributed to the two devices, thereby causing the compromise and the deterioration of measurement accuracy of each of the two devices.